1. Field of the Invention
The invention relates to a spark-ignited internal combustion engine and a method of controlling the spark-ignited internal combustion engine.
2. Description of the Related Art
An available spark-ignited internal combustion engine includes a variable compression ratio mechanism that changes a mechanical compression ratio, and a variable valve timing mechanism that controls a valve closing timing at which an intake valve is closed, wherein during middle or high engine load operation, supercharging is performed using a supercharger, and in addition, during such middle or high engine load operation, as a load applied to the engine decreases, the mechanical compression ratio is increased and the valve closing timing of the intake valve is retarded with an actual compression ratio kept unchanged (see Japanese Patent Application Publication No. 2004-218522, for example).
In a spark-ignited internal combustion engine, the state of the gas in a combustion chamber in the last stage of a compression stroke immediately before combustion, such as the pressure and the temperature of the gas in the combustion engine, significantly affects the combustion. Specifically, in general, combustion is facilitated when the pressure and the temperature of the gas in the combustion chamber in the last stage of a compression stroke is high. However, when the pressure and the temperature of the gas in the combustion chamber are high, knocking can occur. Thus, it is preferable that the pressure and the temperature of the gas in the combustion chamber be kept at optimum values that are maximum values within the range in which knocking does not occur.
When the actual compression ratio is kept constant as in the case of the above-described conventional internal combustion engine, the intake gas supplied into the combustion chamber is always compressed at a constant ratio. However, in this case, the pressure and the temperature of the gas in the combustion chamber in the last stage of a compression stroke significantly vary depending on the specific heat ratio of the intake gas that is supplied into the cylinder, and when the specific heat ratio of the intake gas that is supplied into the cylinder increases, the pressure and the temperature in the combustion chamber in the last stage of a compression stroke accordingly increase. Thus, there is a problem that even when the actual compression ratio is kept constant as in the case of the above-described conventional internal combustion engine, the pressure and the temperature of the gas in the combustion chamber cannot be kept at optimum values.